A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which may be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of extreme UV light (EUV: wavelength 50 nm and less, e.g. between 13 and 14 nm or 11 nm), X-rays, ion beams or electron beams. All of these so-called next-generation radiations undergo absorption in air, so that it becomes desirable to at least partially evacuate the environment in which they are employed. This may be difficult.
A general discussion of the use of EUV in lithographic projection apparatus may be found, for example, in the article by J. B. Murphy et al. in Applied Optics 32 (24), pp 6920-6929 (1993). Similar discussions with regard to electron-beam lithography may be found in U.S. Pat. No. 5,079,112 and U.S. Pat. No. 5,260,151, as well as in EP-A 98201997.8.
An EUV lithograph apparatus is usually formed by a number of sub-systems, such as
a source sub-system, e.g. including an EUV source,
an illumination sub-system, e.g. including an illuminator,
a support structure sub-system, e.g. including a mask table,
a projection sub-system, e.g. including a refractive projection system and
a substrate table sub-system, e.g. including a substrate table or wafer table.
In order to improve the quality of a lithographic apparatus, it is desirable to manage contamination carefully. In order to prevent contamination from migrating through the lithographic apparatus, gas locks may be used. Gas locks may suppress the migration of contamination by providing a counter-flow of gas.
Gas locks may be used to suppress contamination from migrating from one sub-system to another sub-system. Also, gas locks may be used to suppress contamination from migrating from one place (first mini environment) to another (second mini environment) within a sub-system.
According to an example, the substrate table sub-system is separated from the projection sub-system by a gas lock. Such a gas lock is designed to suppress unwanted gas species from migrating from the substrate table sub-system to the projection sub-system. The gas lock is arranged to suppress this migration with a certain suppression factor. According to this example, the gas lock may be arranged to suppress hydrocarbons coming from a substrate W by using a counterflow of hydrogen.
Furthermore, a gas lock may be used to separate the source sub-system (which may contain an aggressive chemical to clean the source mirror as a contaminant) from the illumination sub-system.
So, gas locks are provided to suppress contamination from migrating out of a sub-system, i.e. moving from one part to an other part within a system. It will be understood that also other suppression systems may be used and that a gas lock is just an example of a possible suppression system. Other mechanisms to suppress contamination include cross flows (i.e. a gas lock where the gas flow is directed perpendicular to the direction of the movement of the contaminant), suppression systems using electric, magnetic, gravitational or other force fields, or using temperature, pressure and/or force (for instance in a centrifuge) gradients. In fact, traditional locks (formed by two subsequent doors) may be considered a suppression system as locks are also provided to prevent contamination from migrating.
For some contamination gasses present in the source sub-system (e.g. aggressive chemicals to clean the source mirror or contaminating chemicals like heavy hydrocarbons), the maximum allowable concentrations in the illumination sub-system and projection sub-system are relatively low. The specified maximum partial pressures of the contaminant is often much lower than feasible detection limits.
This may cause problems when the maximal allowable amount of contaminant leaking through the suppression system is below the amount that is detectable using an available detection system. This may result in problematic qualification of the suppression system and/or monitoring of the suppression system, as explained in the paragraphs below.
This may be problematic when one needs to qualify a design and the performance of a suppression system. The performance of the suppression system may not be qualified as the amount of contaminant that is possibly leaking through the suppressor may not be detected. For instance, when a gas lock is used as a suppression system, the amount of allowable contaminant may not be detected ‘upstream’ of the gas lock.
Furthermore, the performance of the suppression system may not be checked during operation. A possible defect in the suppression system may not be detected because the specification may not be measured with sufficient accuracy. For instance, when a gas lock is used as a suppression system and, as an error, the counter flow is only half the desired counter flow, this may not be detected ‘upstream’ of the counter flow, as the amount of contaminant leaking through is not high enough to be detected, although high enough to damage the system. Also, before the supply of an aggressive chemical is opened, it is desirable to ensure that the suppression system is working properly.